1. Field of Invention
The present invention relates to pixel structures for an autostereoscopic display apparatus. Such a display apparatus may be used in televisions, computer monitors, telecommunications handsets, digital cameras, laptop and desktop computers, games apparatus, automotive and other mobile display applications.
2. Description of Related Art
Normal human vision is stereoscopic, that is each eye sees a slightly different image of the world. The brain fuses the two images (referred to as the stereo pair) to give the sensation of depth. Three dimensional (3D) stereoscopic displays show a separate image to each of the eyes corresponding to that which would be seen if viewing a real world scene. The brain again fuses the stereo pair to give the appearance of depth in the image.
FIG. 1 shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 3 on the display plane and a left eye 4 views a left eye homologous point 5 on the display plane to produce an apparent image point 6 perceived by the user behind the screen plane. If light from point 3 is seen by the left eye 4 and light from the point 5 is seen by the right eye 2 then a pseudoscopic image point 21 is produced. Pseudoscopic images are undesirable as they produce visual strain to observers.
FIG. 2 shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 7 on the display plane and a left eye 4 views a left eye homologous point 8 on the display plane to produce an apparent image point 9 in front of the screen plane. Pseudoscopic image point 12 is produced if the right eye 2 can see light from point 8 and the left eye 4 can see light from point 7.
FIG. 3 shows the appearance of the left eye image 10 and right eye image 11. The homologous point 5 in the left eye image 10 is positioned on a reference line 12. The corresponding homologous point 3 in the right eye image 11 is at a different relative position 3 with respect to the reference line 12. The separation 13 of the point 3 from the reference line 12 is called the disparity and in this case is a positive disparity for points which will lie behind the screen plane. Similarly in the left eye image 10, the homologous point 8 is positioned on a reference line 14 while in the right eye image the corresponding homologous point 7 is laterally separated from the reference line 14 by a distance 15 with a negative disparity. Changing from the left eye image 10 to the right eye image 11, the movement of the homologous point 3 is to the right. This corresponds to an orthoscopic image point 6 behind the screen plane, while the movement of the homologous point 7 is to the left, corresponding to an orthoscopic image point 9 in front of the screen plane.
For a generalised point in the scene there is a corresponding point in each image of the stereo pair as shown in FIG. 3. These points are termed the homologous points. The relative separation of the homologous points between the two images is termed the disparity; points with zero disparity correspond to points at the depth plane of the display. FIG. 1 shows that points with uncrossed disparity appear behind the display and FIG. 2 shows that points with crossed disparity appear in front of the display. The magnitude of the separation of the homologous points, the distance to the observer, and the observer's interocular separation gives the amount of depth perceived on the display.
Stereoscopic type displays are well known in the prior art and refer to displays in which some kind of viewing aid is worn by the user to substantially separate the views sent to the left and right eyes. For example, the viewing aid may be color filters in which the images are color coded (e.g. red and green); polarising glasses in which the images are encoded in orthogonal polarization states; or shutter glasses in which the views are encoded as a temporal sequence of images in synchronisation with the opening of the shutters of the glasses.
Autostereoscopic displays operate without viewing aids worn by the observer. In autostereoscopic displays, each of the views can be seen from a limited region in space as illustrated in FIG. 4.
FIG. 4 shows a display device 16 with an attached parallax element 17. The display device 16 produces a right eye image 18 for the right eye channel. The parallax element 17 directs light in a direction shown by the arrow 19 to produce a right eye viewing window 20 in the region in front of the display. An observer places their right eye 22 at the position of the window 20. The position of the left eye viewing window 24 is shown for reference. The viewing window 20 may also be referred to as a vertically extended optical pupil.
FIG. 5 shows the left eye optical system. The display device 16 produces a left eye image 26 for the left eye channel. The parallax element 17 directs light in a direction shown by the arrow 28 to produce a left eye viewing window 30 in the region in front of the display. An observer places their left eye 32 at the position of the window 30. The position of the right eye viewing window 20 is shown for reference.
The parallax element 17 acts as an optical steering mechanism. The light from the left image 26 is sent to a limited region in front of the display, referred to as the viewing window 30. If a left eye 32 is placed at the position of the viewing window 30 then the observer sees the appropriate left eye image 26 produced by the display device 16. Similarly the optical system sends the light intended for the right image 18 to a right eye viewing window 20. If the observer places their right eye 22 in that window then the right eye image 18 produced by the display device 16 will be seen. Generally, the light from either image may be considered to have been optically steered (i.e. directed) into a respective directional distribution.
In this application the term “3D” is used to refer to a stereoscopic or autostereoscopic image in which different images are presented to each eye resulting in the sensation of depth being created in the brain. This should be understood to be distinct from “3D graphics” in which a 3D object is rendered on a two dimensional (2D) display device and each eye sees the exact same image.
The parallax element 17 may be switchable between a state in which it provides a 3D image and a state in which it has substantially no optical effect to allow selective display of 3D and 2D images. In this application the term “2D/3D” is used to refer to a display apparatus in which the function of the optical element can be so switched to enable a full resolution 2D image or a reduced resolution autostereoscopic 3D image.
FIG. 6 shows in plan view a display apparatus comprising a display device 16 and parallax element 17 in a display plane 34 producing the left eye viewing windows 36, 37, 38 and right eye viewing windows 39, 40, 41 in the viewing window plane 42. The separation of the window plane from the display device 16 is termed the nominal viewing distance 43. The viewing window 37 and viewing window 40 in the central position with respect to the display device 16 are in the zeroth lobe 44. Left eye viewing window 36 and right eye viewing window 39 located to the right of the zeroth lobe 44 are in the +1 lobe 46, while left eye viewing window 38 and right eye viewing window 41 located to the left of the zeroth lobe are in the −1 lobe 48.
The viewing window plane 42 of the display apparatus represents the distance from the display device 16 at which the lateral viewing freedom is greatest. For points away from the display plane 34, there are diamond shaped autostereoscopic viewing zones, as illustrated in plan view in FIG. 6. As can be seen, the light from each of the points across is beamed in a cone of finite width to the viewing windows. The width of the cone may be defined as the angular width.
The parallax element 17 serves to generate a directional distribution of the illumination at the viewing window plane 42 at a defined distance 43 from the display device 16. The variation in intensity across the viewing window plane 42 constitutes one tangible form of a directional distribution of the light.
If an eye is placed in each of a pair viewing zones such as left eye viewing window 37 and right eye viewing window 40, then an autostereoscopic image will be seen across the whole area of the display. To a first order, the longitudinal viewing freedom of the display is determined by the length of these viewing zones.
The variation in intensity (or luminance) α 50 across the window plane of a display (constituting one tangible form of a directional distribution of the light) is shown with respect to position x 51 for idealised windows in FIG. 7. The right eye window position intensity (or luminance) function (or distribution) 52 corresponds to the right eye viewing window 41 in FIG. 6, and intensity (or luminance) function 53 corresponds to the left eye viewing window 37, intensity (or luminance) function 54 corresponds to the right eye viewing window 40 and intensity (or luminance) function 55 corresponds to the left eye viewing window 36. The integrated intensity (or luminance) function, 60 is the sum of the intensity (or luminance) from the individual intensity (or luminance) function 52, 53, 54, 55 with respect to the locations of the individual windows 41, 37, 40, 36 and further adjacent windows.
FIG. 8 shows the integrated intensity function 60 with position x 51 schematically for more realistic windows. The right eye window position intensity function 56 corresponds to the right eye viewing window 41 in FIG. 6, and intensity function 57 corresponds to the left eye viewing window 37, intensity function 58 corresponds to the right eye viewing window 40 and intensity function 59 corresponds to the left eye viewing window 36. The ratio of the variation from an integrated nominal intensity function 60 to the nominal intensity in an angular range is termed the angular intensity uniformity (AIU) or alpha (α) function. The nominal intensity function may be for example a flat Luminance function as shown in FIG. 7, a Lambertian function, or some other function with a substantially smoothly varying intensity profile. The AIU may be measured over a limited range of viewing angles, or over the entire angular range of output angles of the respective display.
FIG. 9 shows a further intensity function 61 in which substantially triangular shaped viewing windows are overlapped in order to produce a flat integrated intensity (or luminance) function 60. Advantageously, such windows can provide a robust means by which to reduce non uniformities in the function 60. Further such windows reduce image flipping artefacts in which the image content appears to rapidly change from one view to another in multi-view displays, causing an apparent rotation of the image to an observer.
Several 3D artefacts can occur due to inadequate window performance, particularly for overlapping windows. Pseudoscopic images occur when light from the right eye image is seen by the left eye and vice versa. This is a significant 3D image degradation mechanism that can lead to visual strain for the user. Overlapping windows are seen as image blur, which limits the useful amount of depth that can be shown by the display. Additionally, poor window quality will lead to a reduction in the effective viewing freedom of the observer. The optical system is designed to optimise the performance of the viewing windows.
In displays with multiple views, adjacent windows contain a series of view data. As an observer moves laterally with respect to the display device, the images seen by each eye vary so that the appearance of a 3D image is maintained. Human observers are sensitive to variation in luminance as they move with respect to the display. For example, if the integrated intensity (or luminance) function 60 varies by more than 0.5%-5% of the maximum, then the display will appear to flicker. Thus it is desirable to minimise the variation of the integrated intensity (or luminance) function 60. As the function varies with the viewing angle, the uniformity of the function may be referred to as the angular intensity uniformity (AIU) which is an important performance parameter.
The respective images are displayed at the display plane 34, and observed by an observer at or near the viewing window plane 42.
There will now be discussed some known techniques for improving the AIU of a display.
One type of prior art pixel configuration for autostereoscopic display apparatus uses the well known stripe configuration as shown in FIG. 10a as used for standard 2D displays. The pixels apertures 62 are arranged in columns of red pixels 65, green pixels 67 and blue pixels 69. To generate an autostereoscopic display, a parallax element 172 such as a lenticular array is aligned with groups of color pixels 65, 67 and 69 as shown. The cusp 71 between the lenses of the array is one example of the geometric axis of the array of parallax elements.
The parallax element 172 may be slanted so that the geometric axes of the optical elements (e.g. lenses in the case of a lenticular array) of the parallax element 172 are inclined to the vertical column direction of the pixel apertures 62, as described for example in U.S. Pat. No. 3,409,351 and U.S. Pat. No. 6,064,424. Such an arrangement enables overlapping of windows, similar to that shown in FIG. 9, that results in a better uniformity of the integrated intensity (or luminance) function 60 of intensity compared to a parallax element in which the geometric axes of the optical elements are parallel to the vertical column direction of the pixel apertures.
Herein, a line parallel to the geometric axes of the optical elements of a parallax element is termed a “ray line”, being a line along which rays of light are nominally (ignoring aberrations) directed from a display device to the same relative horizontal position in the viewing window plane at any one vertical position in the viewing window plane, rather than being the direction of a ray of light. FIG. 10a further shows the inclined orientation of the ray lines 64 and the geometric axes of the optical elements of the parallax element 172 with respect to the pixel apertures 62. Such an arrangement will generate windows that are tilted with respect to the vertical such that the view data will appear to change as the observer moves vertically.
FIG. 10a further includes a graph of the resultant overlap (or intersection) of ray lines 64 with the pixel aperture function providing an intensity function termed herein the zeta (ζ) function 73. The zeta (ζ) function 73 varies with position y 49 in the pixel plane. As will be described below, this is related to the window intensity function alpha (α) 50 at positions x 51 across the window plane 42.
For ease of understanding, the positions y 49 where a ray line 64 crosses the function 75 correspond to horizontal position y 49 into which light is directed from the ray line 64. The intensity function 75 of the zeta (ζ) function 73 has an intensity which is generally flat but which has peaks 74 whose origin has been appreciated as follows.
The zeta (ζ) function at each given position y 49 can be determined by measuring the total intersection length 66, 68, 70, 72 (shown in bold lines) of the ray lines 64 corresponding to that position y 49 across adjacent pixel apertures 62. This is because, in operation, the parallax element 172 collects light from a ray line 64 and directs it all towards a position in space where that light is observed by the viewer.
In fact an eye receives light from a bundle of ray lines 64 from a region, or spot at the pixel plane due to the pupil size, lens aberrations and lens focus condition so the actual window integrated intensity function alpha (α) 60 observed is a convolution of the zeta (ζ) function 73 with the spot function, sigma (σ), but this will still have similar peaks. Thus as the integrated intensity function 60 varies as the total intersection length varies due to the ray line 64 varyingly covering different amounts of the pixel apertures 62 and the gaps therebetween. In particular, the intensity function 75 includes elevated levels where the total intersection length is high because the ray line 64 intersects more of the pixel apertures 62 in the corner thereof.
As can be seen, the total intersection length 66, 68, 70, 72 can include contributions from two adjacent pixel apertures 62. While these adjacent pixels may have two different colors, each will have a corresponding pixel of the same color in the unit cell structure of the 3D image. So, the adjacent pixels can conveniently be used to form an understanding of the total intersection length within a single color.
In some known systems with non-uniform zeta (ζ) intensity function 75 where the parallax element 172 is a lenticular array, the lenses may be defocussed in order to smooth the alpha (α) integrated intensity function 60, effectively by providing an average of the different intersection lengths 66 of different ray lines 64. However, such an approach creates an increased overlap between the 3D windows and results in increased levels of image blur, reduced useful depth and increased pseudoscopic images. It is therefore desirable to maintain a high AIU without increasing the defocus of the lenses.
WO-2007/031921 discloses a technique by which the features such as peaks 74 in the intensity function 75 are reduced by means of a pixel cut-out 76 as shown in FIG. 10b. The cut-out 76 compensates for the increased intersection which otherwise occurs in the corner of the pixel aperture 62 reducing the total intersection length 78 for those ray lines 80 and thereby flattening the zeta (ζ) intensity function 75. However, such an arrangement cannot be used to compensate the output of wide viewing angle displays, as will be shown below.
Conventional Liquid Crystal Display (LCD) panels such as twisted nematic Liquid Crystal Display (TN-LCD) with homogeneous alignment use substantially rectangular pixel aperture shapes in which the whole of the pixel operates as a single domain such that the angular contrast properties of the optical output are substantially constant for each part of the pixel. Such pixels are well suited to the rectangular cutout approach to improve uniformity of integrated intensity function 60. However, such panels suffer from significant variations of contrast with viewing angle due to the restrictions of the optical performance of a single liquid crystal alignment domain within the cell. To compensate for such viewing angle effects, one approach is to use Vertical Aligned (VA) LC materials in combination with multiple domain structures and further complex alignment modification techniques. In this case each pixel comprises plural domains having different alignments of the liquid crystal molecules. The contrast properties with viewing angle of the display are determined by the addition of contrast properties from the individual domains.
One approach for improving the AIU is for the display to implement a radially symmetric mode. In this case, the apertures (displaying area) of the pixels of the spatial light modulator comprise an alignment feature, such as a bump feature, that provides radially symmetric alignment of the molecules of the liquid crystal. In general such a display has the capability of improving the angular characteristics of the display apparatus.